Tomography may refer to imaging by sections or sectioning, through the use of any kind of penetrating wave. Tomography may be used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.
Tomosynthesis is a method for performing high-resolution limited-angle tomography. Tomosynthesis dates back to the 1930s, where film-based radiography systems were used, with long exposure from a moving X-ray tube. Today, tomosynthesis combines digital image capturing and processing with simple tube/detector motion as used in conventional computerized tomography (CT). Although there are some similarities to CT, it is a separate technique. In CT, the source and detector make at least a 180-degree rotation about the subject, obtaining a complete set of data from which images may be reconstructed. In digital tomosynthesis, only a limited rotation angle (e.g., 15-60 degrees) may be used, with a relatively small number of discrete exposures (typically 10-80). This incomplete set of projections may be digitally processed to yield images, similar to conventional tomography but with a limited depth of field. However, because the image processing is digital, a series of slices at different depths and with different thicknesses may be reconstructed from the same acquisition, saving both time and radiation dose.
CT scans are typically performed by dedicated, high-end systems, with fast continuous rotations and with high-accuracy encoders for exact positioning of the system components. Tomosynthesis, on the other hand, is traditionally performed by simpler, less costly system, with compromised mechanical accuracy and robustness. Therefore, radiopaque objects (“fiducials”) are commonly used in the tomosynthesis scans for better mutual alignment of the various views. Fiducial are generally fully opaque objects and therefore their trace may not be removed from the resulting image.
In the most general system, where both the detector and the X-ray source move freely, there are nine degrees-of-freedom (DOFs): three for the position of the detector center, three angular DOFs for the detector orientation in space and three for the X-ray source. Typically, a less general geometry is used, with up to eight DOFs, while one DOF is being held fixed. For example, the distance between the source and the detector might be kept constant. For that purpose, at least four fiducial points have to be used, each contributing two independent measures of x- and y-coordinates. However, more fiducials are typically used.
U.S. Pat. No. 5,359,637 to Webber discloses a self-calibrating tomosynthetic x-ray system. A calibrated device for recording radiographic images of a selected object irradiated by a source of radiation includes a first radiolucent radiographic recording medium in the form of a CCD device for recording a first projected radiographic image of the selected object. A second radiographic recording medium in the form of a CCD device is supported in fixed generally parallel position relative to the first radiographic recording medium to permit radiation from the source to pass through the first radiographic recording medium and to impinge upon the second radiographic recording medium for recording a second projected radiographic image of the selected object. A radiopaque fiducial reference in the form of a grid is supported in fixed position generally between the first and second radiographic recording mediums to permit a projected image of the radiopaque fiducial reference to be recorded on the second radiographic recording medium. Projected radiographic images of the object and the fiducial reference are then recorded at different arbitrary relative positions between the source of radiation and the object, the fiducial reference, and the recording mediums. An image of a selected object at a selected slice position through the object is synthesized from selected projected radiographic images of the object and the fiducial reference recorded by the calibrated device.
U.S. Pat. No. 6,888,924 to Claus et al. discloses geometry of a tomosynthesis system including a detector and an x-ray source, which is determined using fiducial markers with non-determined positions. The geometry is determined by arbitrarily identifying at least two markers within an imaged volume, at different relative distances between the detector and the x-ray source, without having projections located on a straight line for all different source positions, and locating the projections of the markers within at least two images acquired of the imaged volume. The at least two images correspond to different positions of a focal spot of the x-ray source.
US Patent Application Publication No. 2012/0014498 to Akahori discloses a radiographic imaging apparatus which includes: a radiation source for applying radiation to a subject and at least one marker; a detecting unit for detecting the radiation transmitted through the subject; and an image obtaining unit for moving the radiation source relative to the detecting means, applying the radiation to the subject from a plurality of radiation source positions provided by the movement of the radiation source, and obtaining a plurality of images corresponding respectively to the radiation source positions. The apparatus further includes a radiation source position obtaining unit for obtaining positional information of each radiation source position of interest relative to a reference radiation source position among the radiation source positions based on at least one marker image contained in each of a reference image obtained with the reference radiation source position and an image of interest obtained with the radiation source position of interest.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the figures.